This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-205147 filed on Jul. 6, 2000.
1. Field of the Invention
The present invention relates to a voltage control unit used for a vehicular AC generator.
2. Description of Related Art
JP-A-55-127849 and JP-A-6-284598 disclose to detect, during rotation of a revolving-field type alternator, an AC voltage (also called armature coil induced voltage with residual magnetism) which is induced in a field coil as a result of a magnetic flux being modulated with rotation of field poles, the magnetic flux remaining in a core and interlinked with the armature coil, and thereby detecting the rotation of the alternator, i.e., start-up of an engine.
JP-A-3-215200 discloses to detect a potential difference between two-phase voltages out of polyphase AC voltages which are induced by interlinkage of a magnetic flux with a polyphase armature core, the magnetic flux remaining in rotating magnetic poles as constituents of a rotating-field type alternator, thereby detecting the start of rotation.
According to the above conventional power generation detecting prior arts, IG cable is omitted and in the case where an engine does not start despite turning ON of an IG switch, the alternator is not energized.
However, in the above rotation detection based on the armature coil induced voltage with residual magnetism, the residual magnetism of core often becomes small and hence the armature coil induced voltage with residual magnetism is small, so that it is desired to improve the detection reliability.
Similarly, the field coil induced voltage with residual 10 magnetism, which is cased by residual magnetism of the armature core, is influenced by magnetism decline. This is more remarkable.
Generally, since the inductance of a field coil is large, a current return diode is connected in parallel with the field coil to inhibit the occurrence of overvoltage. This means that even when a switching transistor connected in series with a field coil is turned ON, allowing a field current to flow through the field coil, and is thereafter turned OFF, the field current continues to flow in the field coil through the current return diode for as long as several hundred seconds while attenuating. Further, even when fuel injection for an engine is stopped upon turning OFF of the switching transistor and hence upon stop of external power supply to the field coil, the engine and the vehicular AC generator continue to rotate for a predetermined period of time by the force of inertia while slowing down. As a result, various portions of the armature core are magnetized alternately by field poles of opposite polarities of which magnetic field intensity attenuate gradually with attenuation of the field current. Under an AC demagnetizing action which is known well in the field of magnetic recording, the residual magnetism disappears, as shown in FIG. 6.
It has experimentally turned out that when external power supply to a field wirding is stopped under the conditions of time constant xcfx84 of the field coil: 200 msec, the number of magnetic poles: 16 and the number of rotation: 2000 rpm, a field current continues to flow while attenuating exponentially for subsequent 200 msec or so. At this time, an attenuated alternating field Hs of 266.66 Hz is applied to the armature core, so that the residual magnetism of the armature core remarkably decreases.
In the detection of an armature coil induced voltage with residual magnetism, residual magnetism of a field core is important. But, since it is influenced by residual magnetism of an armature core and a current field of the armature core, it so much decreases, thereby giving rise to the problem of a lowering in residual magnetism like the armature core.
An object of the present invention is to provide a voltage control unit for a vehicular AC generator preventing the attenuation of residual core magnetism in the vehicular AC generator, thereby increasing an induced voltage with residual magnetism.
According to the present invention, stop of external power supply to a field coil caused by the stop of power supply to a control circuit is done at least after the stop of a rotor, whereby it is possible to prevent a field current from being attenuated by the foregoing AC demagnetizing action which is attributable to interruption of the power supply to the control circuit before stop of the rotor. As a result, it is possible to prevent the occurrence of the AC demagnetization phenomenon, thereby preventing the decrease of residual magnetism of a core, especially the armature core.